1. n. The basic unit of
computation. What every hacker wants more of (noted hacker Bill Gosper
described himself as a “cycle junkie”). One can describe an
instruction as taking so many clock
cycles. Often the computer can access its memory once on every
clock cycle, and so one speaks also of memory
cycles. These are technical meanings of
cycle. The jargon meaning comes from the
observation that there are only so many cycles per second, and when you are
sharing a computer the cycles get divided up among the users. The more
cycles the computer spends working on your program rather than someone
else's, the faster your program will run. That's why every hacker wants
more cycles: so he can spend less time waiting for the computer to respond.

2. By extension, a notional unit of human
thought power, emphasizing that lots of things compete for the typical
hacker's think time. “I refused to get involved with the Rubik's
Cube back when it was big. Knew I'd burn too many cycles on it if I let
myself.”